This invention relates to a process for rendering surfaces permanently water wettable and to water wettable articles of manufacture thus-produced.
It is sometimes desirable that a surface of an article of manufacture exposed to water be water wettable. The term "water wettable" as used herein means the ability of the surface to retain a substantially unbroken film of water thereon, preferably for at least 10 seconds and more preferably for at least 30 seconds after removal from water, e.g., after being immersed in water or flushed with a stream of water. This property is desirable because a water wettable surface of an article of manufacture exposed to water condensation has less of a tendency to drip droplets of water therefrom than a surface which is not water wettable. Instead of dripping in a random fashion from the surface, the water migrates downwardly along the surface in a predictable path and if droplets form, they can be collected at a predetermined desired point. An example of articles of manufacture where water wettability is a necessary property is an aluminum refrigerator evaporator, whose external surfaces conventionally are rendered permanently water wettable by metal anodizing, and the external surfaces of other evaporators and heat exchangers whose efficiency is dependent on the surfaces being wet with water. Other examples of articles of manufacture in which water wettability is a desirable property are those which are maintained below ambient temperature in a humid atmosphere and which are therefore susceptible to condensation and dripping, e.g., plastic, for example, PVC, polyethylene, and metal, for example, aluminum, copper and iron, cold water pipes and sheets forming the interior surfaces of refrigerators.
Another example of articles of manufacture where water wettability is a desirable or necessary property are those formed of or having a coating on the surface thereof of a hydrophobic polymer, which are to be coated with an aqueous coating composition. It is difficult and sometimes impossible to apply a uniform coating of a desired aqueous coating composition to such hydrophobic surfaces. Therefore, the surface of the polymer must be physically or chemically altered to render it receptive to the selected aqueous coating composition.
It is relatively simple to render metal surfaces temporarily water wettable. For example, acidic aqueous colloidal dispersions of .alpha.-alumina are used commercially to render surfaces temporarily water wettable. See Trade Literature "Dispal.RTM. Alumina", Continental Oil Company, Market Development Dept., Saddle Brook, N.J. Although that literature states that the durability of the alumina film is increased by incorporating a small percentage, e.g., 2%, of acrylic emulsion in the dispersion, the wettability imparted by the alumina film is still relatively impermanent and moreover, is far less effective than anodizing in protecting the aluminum substrate from corrosion and/or staining. Therefore, this method of rendering aluminum articles of manufacture water wettable is unsatisfactory for products which are subjected for long periods of time to conditions which could stain or corrode the surface of the aluminum, as is the case of the interior aluminum surfaces of refrigerator boxes and the exterior surfaces of refrigerator and air conditioner evaporators.
The coating of aluminum with polymeric coatings is, of course, well known in the prior art. U.S. Pat. No. 3,464,854 discloses a coating composition which can be applied to aluminum surfaces, consisting of an eposy resin binder in aqueous dispersion with a major amount of particulate filler, e.g., alumina. Additionally, an amine curing agent is provided in the composition. U.S. Pat. No. 3,468,753 describes a method for applying a layer of co-polymer ethylene and unsaturated carboxylic acids such as acrylic and methacrylic acid, which layer may include a filler material, to a metal sheet, e.g., aluminum.
The electrodeposition of acidic polymers on metal substrates is well known. See, e.g., U.S. Pat. Nos. 3,230,162; 3,463,714; 3,494,847; 3,507,765; 3,532,613; 3,556,972; 3,575,902; 3,741,923; and 3,755,119.
The hydrophilization of hydrophobic polymers is also known in the prior art. U.S. Pat. No. 3,935,342 discloses a process wherein hydrophobic polymers are treated by contacting them with silylated acrylate or methacrylate monomers, polymerizing the monomer and then hydrolyzing the polymer to break the siloxy bonds and produce free hydroxy groups. The product can be used in a large number of coating applications. See, Col. 1, lines 45-53.
It is an object of this invention to provide a process for rendering non-polar or hydrophobic surfaces permanently water wettable. Another object is the provision of novel articles of manufacture having a surface rendered permanently water wettable according to the process of this invention. Other objects will be apparent to those skilled in the art to which this invention pertains.